Operation of lean burn engines, e.g. diesel engines and lean burn gasoline engines, provide the user with excellent fuel economy and have very low emissions of gas phase hydrocarbons and carbon monoxide due to their operation at high air/fuel ratios under fuel lean conditions. Diesel engines, in particular, also offer significant advantages over gasoline engines in terms of their durability and their ability to generate high torque at low speed.
From the standpoint of emissions, however, diesel engines present problems more severe than their spark-ignition counterparts. Emission problems relating to particulate matter (PM), nitrogen oxides (NOx), unburned hydrocarbons (HC) and carbon monoxide (CO). NOx is a term used to describe various chemical species of nitrogen oxides, including nitrogen monoxide (NO) and nitrogen dioxide (NO2), among others. NO is of concern because it is believed to under a process known as photo-chemical smog formation, through a series of reactions in the presence of sunlight and hydrocarbons, and NO is a significant contributor to acid rain. NO2, on the other hand, has a high potential as an oxidant and is a strong lung irritant. Particulates (PM) are also connected with respiratory problems. As engine operation modifications are made to reduce particulates and unburned hydrocarbons on diesel engines, the NO and NO2 emissions tend to increase.
Effective abatement of NOx from lean burn engines is difficult to achieve because high NOx conversion rates typically require reductant-rich conditions. Conversion of the NOx component of exhaust streams to innocuous components generally requires specialized NOx abatement strategies for operation under fuel lean conditions
Selective catalytic reduction (SCR), using ammonia or ammonia precursor as reducing agent is believed to be the most viable technique for the removal of nitrogen oxides from the exhaust of diesel vehicles. In typical exhaust, the nitrogen oxides are mainly composed of NO (>90%), so the SCR catalyst favors the conversion of NO and NH3 into nitrogen and water. Two major challenges in developing catalysts for the automotive application of the ammonia SCR process are to provide a wide operating window for SCR activity, including low temperatures of from 200° C. and higher and improvement of the catalyst's hydrothermal stability for temperatures above 500° C. As used herein hydrothermal stability refers to retention of a material's capability to catalyze the SCR of NOx, with a preference for the retention to be at least 85% of the material's NOx conversion ability prior to hydrothermal aging.
Metal-promoted zeolite catalysts including, among others, iron-promoted and copper-promoted zeolite catalysts, where, for instance, the metal is introduced via ion-exchange, for the selective catalytic reduction of nitrogen oxides with ammonia are known. Iron-promoted zeolite beta has been an effective catalyst for the selective reduction of nitrogen oxides with ammonia. Unfortunately, it has been found that under harsh hydrothermal conditions, such as reduction of NOx from gas exhaust at temperatures exceeding 500° C., the activity of many metal-promoted zeolites, such as Cu and Fe versions of ZSM-5 and Beta, begins to decline. This decline in activity is believed to be due to destabilization of the zeolite such as by dealumination and consequent loss of metal-containing catalytic sites within the zeolite.
To maintain the overall activity of NOx reduction, increased levels of the washcoat loading of the iron-promoted zeolite catalyst must be provided. As the levels of the zeolite catalyst are increased to provide adequate NOx removal, there is an obvious reduction in the cost efficiency of the process for NOx removal as the costs of the catalyst rise.
In some SCR systems, particularly heavy duty diesel (HDD), controlling secondary pollutant N2O emitted from the SCR system has become more important. Additionally, certain existing catalysts, such as copper promoted zeolites (e.g Cu-SSZ-13), tend to produce unacceptably high N2O emissions. Because N2O is a greenhouse gas and emissions regulations are becoming increasingly stringent, there is a need for systems that reduce the amount of N2O emitted from SCR systems.